docs: restructure navigation into five top-level sections (Start / Build / Operate / Security / Internals)

[abrichr]

What

Reorganizes docs.openadapt.ai into the review's five top-level sections and relinks the nav accordingly. This is an organize-what-exists change — no page content was rewritten and no new docs were invented.

Five-section structure

Section	Scope	Pages moved in
Start	install, first success, quickstart	get-started overview, desktop/install, first-workflow, what-you-get
Build	record → compile → lint → certify, effect verifiers, identity	concepts overview + all how-to guides, demonstration-compiler, workflow-ir, record-your-app, induce-a-program, multi-trace-induction, parameters-and-secrets, capability-ladder, policy-and-certification, policy-and-certify, effect-verification, identity-gate
Operate	run, halt/teach, on-prem, hosted lifecycle, monitoring	run-a-deployment, run-reports, halt-learn-loop, self-healing, durable-runtime, deploy-on-prem, vlm-appliance, hosted, connect-to-cloud, troubleshooting
Security	LIMITS / claims, trust boundary, PHI	what-works-today (qualification evidence), regulated-execution, security-and-data-handling, security-review
Internals	architecture, package registry, contributing/governance	substrate-model, deployment-matrix, backends, reference/* (CLI, compatibility, deployment-config, bundle-format, configuration), ecosystem catalog, documentation-governance, what's-new, changelog

Notes

• Pages keep their existing on-disk paths, so every cross-link, the substrate-evidence copy tests, and the product-docs contract markers are preserved.
• scripts/validate_docs.py's package-catalog guard previously required the ecosystem catalog under a section literally named Reference; it now accepts Reference or Internals, keeping the no-package-first-IA intent while matching the new section name.
• The "seeking design partners" honesty and scoped substrate evidence are untouched; no customer names introduced.

Gates

• mkdocs build --strict — pass
• python scripts/validate_docs.py — "Validation passed!"
• pytest tests/ — 64 passed